Horsefly Bite

What is a horsefly bite?

A horsefly (Tabanus spp.) is a large fly that commonly lives near bodies of water and where it can easily access mammals. It gets its name because it’s a frequent pest of horses and other mammals (including humans).

Female horseflies bite to feed on blood. They have well-developed mouthparts for tearing skin and a sponge-like tongue that soaks up blood. This is different from mosquitos, for example, which pierce your skin and then suck blood. This also explains why you can typically feel a horsefly bite as it’s happening but not a mosquito bite.

Horsefly bites are painful, but they’re usually not harmful to humans. But it’s possible to have an allergic reaction to a bite or develop an infection from it. In very rare cases, a horsefly can pass on a bacterial infection called tularemia through a bite.

What does a horsefly bite look like?

A horsefly bite typically looks like a raised bump of red or discolored skin. You’ll likely only have one or a couple of bites. These bites are painful, so you’ll likely swat the horsefly away before it can bite again.

What do horseflies look like?

Horseflies have a gray or blackish body and are 10 to 30 millimeters long. They also have large eyes and stout bodies. There are over 160 species of horseflies.

Adult horseflies are fast fliers that most often attack moving, dark objects because they see these as food sources. They exist in most areas of the world except for some islands and the polar regions.

How common are horsefly bites?

Female horseflies are relentless in their search for blood, so horsefly bites can be quite common if they live near you. If you live on or near a farm with livestock, you’ve probably encountered many horsefly bites. However, horseflies are more likely to bite cows and horses than humans.

Horseflies are most active in warm and wet weather.

What are the symptoms of a horsefly bite?

The first sign of a horsefly bite is pain. You may see a horsefly on you as you feel the pain from the bite. After the bite, you’ll likely have a raised, red or discolored area of skin that may be sensitive to touch. You may also notice a little bit of blood.

In most cases, horsefly bites heal and go away within a matter of days.

What are the possible complications of a horsefly bite?

It’s rare to have complications related to a horsefly bite, but they can include:

• Allergic reaction.
• Infection.
• Infection transmission.

Horsefly bite allergic reaction

In extreme cases, bites can cause an allergic reaction. Horseflies release a protein when they bite you that slows blood clotting. Some people may develop an allergic reaction to this protein.

Symptoms of an allergic reaction from a horsefly bite include:

• Skin redness or discoloration that spreads beyond the bite.
• Itchy skin.
• Hives.
• Swelling around the bite.

Minor allergic reactions typically go away on their own with time. If these symptoms continue to get worse, talk to your healthcare provider.

Signs of a severe allergic reaction (anaphylaxis) include:

• Swelling of your lips, tongue or throat.
• Shortness of breath (dyspnea), trouble breathing or wheezing.
• Dizziness and/or fainting.
• Stomach pain, vomiting or diarrhea.

Anaphylaxis is life-threatening. Call 911 (or your local emergency service number) or go to the nearest hospital if you have symptoms of anaphylaxis.

Infected horsefly bite

It’s possible to develop an infection after a horsefly bite. This is more likely to happen if you scratch at the bite and don’t keep it clean. Symptoms of a skin infection (cellulitis) include:

• Fever and chills.
• Swelling, skin discoloration or red streaking around the bite area.
• A fluid-filled blister around the bite.
• Warmer-than-usual skin around the bite.
• Pus or drainage from the bite.
• Swollen lymph nodes, especially those closest to the bite area.
• Fatigue.

You should see a healthcare provider if you have signs of an infection. Infections need treatment with antibiotics.

Infection transmission from horsefly bite

In rare cases, horseflies can transmit (pass on) a bacterial infection called tularemia through bites. This mainly affects wild rodents, but ticks, biting flies and mosquitoes can pass it on to humans.

Symptoms of tularemia develop three to five days after exposure. They typically come on quickly after that time span and can include:

• Fever, chills and sweating.
• Headache.
• Joint stiffness and muscle pain.
• A bite mark that becomes a sore (ulcer).
• Shortness of breath.
• Weight loss.

See a healthcare provider if you have these symptoms.

Why do horseflies bite?

Female horseflies need blood to stimulate egg production for reproduction. They can be relentless and aggressive as they look for mammals (like horses, cows and humans) to bite.

Male horseflies don’t produce eggs, so they don’t need blood. They mainly feed on pollen and plant nectars.

How should I treat a horsefly bite?

If you get a horsefly bite, wash the area with soap and water to help lower the chance of infection.

Most bites get better in a few days. But there are some things you can do at home to ease your symptoms, including:

• Putting an ice pack wrapped in a cloth on the bite for at least 20 minutes. This can help with swelling.
• Keeping the area raised, if you can.
• Taking painkillers like NSAIDs (nonsteroidal anti-inflammatory drugs) if you’re experiencing pain from the bite. Be sure to follow the instructions for taking the medication.
• Taking an antihistamine to relieve any itching. Be sure to follow the instructions for taking the medication.

Try to avoid picking at or scratching the bite area.

How can I prevent horsefly bites?

Some steps you can take to help prevent horsefly bites include:

• Avoid marshlands and beaches or other open areas next to woodlands, especially when it’s not windy.
• Wear light-colored long pants and long-sleeved shirts, as well as a hat when you’re outside.
• Apply bug sprays on your skin that contain DEET, picaridin or lemon-eucalyptus.

If you notice increasing numbers of horseflies in your area, consider talking to a pest control specialist.

When should I see my healthcare provider about a horsefly bite?

If you develop signs of an allergic reaction, infection or tularemia after a horsefly bite, see your healthcare provider.

A note from Cleveland Clinic

Horsefly bites are a painful, pesky reality for many people who live in areas with warm and wet weather. In most cases, horsefly bites are ultimately harmless. But, like all insect bites and stings, you should keep the area clean and avoid scratching it to prevent infection. Contact your healthcare provider if your symptoms last longer than a few days or if you develop an allergic reaction.